The invention relates to electronic security systems and more particularly to systems that use a remote electronic unlocking device known as a proximity key. Pertinent to the invention are several prior art devices and systems which teach electronic alarm apparatus for monitoring the presence or absence of objects within a given area by monitoring for a coded radio frequency signal. Such devices may contain a monitor including a logic timing circuit for activating a receiver for receiving the radio frequency signal, a check circuit for comparing the received signal with a stored identification code and an audio logic circuit for providing audio and visual indications upon failure of receiving a given number of consecutive transmissions from a given transmitter. The prior art describes an electronic lock actuated by an electronic key. The key comprises a plurality of different resistive elements and a signal generator having a plurality of different timing periods. The key further has a signal transmitter. The electronic lock means comprises a lock and a signal receiver. The lock further has a comparator which includes a plurality of different resistive means. The comparator compares each consecutive timing period with the timing period determined by one of the resistive means. The lock further has an activating element for the lock in the event the consecutive timing periods of said signal match the timing periods determined by the plurality of resistive means. Another prior art device shows an electronic security system and an electronic proximity key for use therein in which a multiple tiered distributed architecture is used to rapidly and flexibly provide ingress and egress through a plurality of electronic locks. In the event of loss of communication with the central processor, the system will continue to provide alarm monitor processing. An improved proximity key for actuating the security system is disclosed which includes coupling coils that are integrally formed as part of the integrated circuit lead frame associated with the coding circuity of such a key. These configurations may be found in the pertinent prior art described in U.S. Pat. No. 4792796, U.S. Pat. No. 4727369, and U.S. Pat. No. 4713660. None of the prior art shows the use of a field disturbance sensor using microwave emission to overcome the FCC prohibition to continual emission in the radio frequency range for devices falling in the class pertinent to the current invention. The prior art does not disclose the use of acoustic wave energy (either audible or ultrasonic) in addition to or in lieu of electromagnetic wave energy The prior art does not disclose the use of acoustic communication links in order to measure the distance between the base unit and the proximity key transceiver, which is available through simple circuitry because of the relatively slow speed of sound, and which is advantageous in preventing the system's disarmament unless the key is precisely within some preselected definition of proximity. The prior art does not disclose the compact, inexpensive to manufacture and highly efficient circuit approach presented in the present invention for achieving the objectives disclosed. Therefore, there exists a need for a proximity sensor which can sense the presence of an intruder without continual radio frequency transmission. The present invention fulfills these needs in a relatively inexpensive fashion and provides further related advantages as described and shown herein.